Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 437

Figure 14.19 Laboratory direct shear test apparatus (Erban 1986)
In a direct shear test, the specimen is first subjected to the specified normal stress level
to allow excess pore pressure in the rock specimen or discontinuity filling to dissipate
before shearing. The corresponding normal displacement of the specimen is recorded
as a function of time. Consolidation can be considered as complete when the rate of
normal displacement is less than 0.05 mm/min (ISRM 1975a). The steel wire or tape
still holding the specimen's two halves together is thus unloaded and can be removed.
Subsequently, the shear force is applied continuously with a constant rate of shear dis-
placement. According to DGEG (1988), a displacement rate of 2 mm/min has proven
to be reasonable. The shear force and, if measured, normal and lateral displacements
should be recorded during shearing at short time intervals or continuously. After reach-
ing peak strength, the rate of shear displacement should be reduced to 0.02 up to at
most 1 mm/min until residual strength is reached (ISRM 1975a).